For a power conversion circuit of a drive motor for a car, a switching element such as power MOSFET is used. The switching element, when a voltage exceeding the withstand voltage is applied, is damaged, so that a means for suppressing an overvoltage is installed.
For example, in Japanese Patent Laid-Open No. 1996-23700, an overvoltage suppression means in a power generation source for rectifying a generated voltage from a generator and obtaining a DC output voltage is described. In the example of this patent application, a voltage detector and a switchgear are installed and when the voltage detector detects an overvoltage of DC output, the generated voltage is short-circuited and discharged by the switchgear.
In Japanese Patent Laid-Open No. 2002-17098, in a motor controller for controlling a motor, a means for suppressing an overvoltage by turning on a switching element of a power conversion circuit according to the rotation position of the motor when the overvoltage is generated and connecting a plurality of phases of the motor in series is proposed.